In conventional image forming apparatus such as electrophotographic copying machines, a surface of a photoreceptor (a charged member) is positively or negatively charged uniformly by a corona discharge device. In a subsequent exposure process, certain points of the surface are selectively discharged to form an electrostatic latent image. Then, a developer supplying device with a predetermined amount of developing bias applied supplies developer to the surface of the photoreceptor, so that the latent image is visualized, i.e., developed.
Some image forming apparatus using the corona discharge method are provided with a combined developing/cleaning device. Such image forming apparatus uses a toner scattering process, instead of a dedicated cleaning device. In the toner scattering process, an electrically conductive brush scatters residual toner particles remaining on the photoreceptor after a preceding transfer process. Also, such apparatus adopts a developing process using magnetic toner. See Japanese examined Patent Application No. H06-50416, p. 3, left column, lines 4 to 7.
The combined developing/cleaning device allows for downsizing of such apparatus. However, the corona discharge device provided in such apparatus is easily affected by environmental factors such as humidity or dust. Also, the corona discharge process involves ozone emissions which have an unpleasant odor and possible harmful effects on human health.
One solution to the foregoing problems is a contact charging method in which a surface of a charged member (photoreceptor drum) is charged by direct contact with a conductive member (charging roller) to which a direct-current voltage with an alternating-current voltage superposed is applied.
The contact charging method, however, causes problems as described below. In an image forming apparatus using the contact charging method, a conductive member (charging roller) becomes in direct contact with a surface of a charged member (photoreceptor drum). Accordingly, when there are relatively hard particles, such as toner carriers, on the surfaces of the charged member and the conductive member, the particles scratch the surfaces when the surfaces become in contact with each other. Also, foreign particles which adhere to a portion of the surface of the conductive material (charging roller) cause a corresponding portion of the surface of the charged member (photoreceptor drum) to be non-uniformly charged.
To solve the foregoing problems of the contact charging method as well as to achieve the greatest advantage thereof, i.e., no ozone emission, there has been proposed a noncontact charging method in which a charging member is positioned in proximity to (thus, out of contact with) a photoreceptor. See FIG. 1 of JP-H05-307279-A, or FIG. 1 of JP-H07-301973-A.
Application of the noncontact charging method to an image forming apparatus provided with a two-component developing device has also been proposed. See paragraph [0019], and FIG. 1, of JP-2001-188403-A. In the apparatus, a narrowest gap between a discharging surface of a charging member and a photoreceptor is rendered larger than diameter of a toner carrier particle. This prevents a toner carrier, or toner carried on the toner carrier, from getting stuck in the gap. Thus, the toner carrier is prevented from scratching or contaminating the surfaces of the photoreceptor and the charging member.
However, the apparatus as disclosed by JP-2001-188403-A does not have a combined developing/cleaning device such as disclosed by Japanese examined Patent Application No. H06-50416. Consequently, the apparatus tends to grow in size and to require a high supply voltage. Also, since the narrowest gap between the surfaces of the charging member and the photoreceptor is rendered larger than the diameter of the toner carrier particle, an extra amount of voltage is required for charging the photoreceptor.
Further, if the gap is rendered smaller than the diameter of the toner carrier particle to solve the problem, a voltage applied to the charging roller is reduced. However, fluctuations in gap width may have greater effects, and therefore the gap width has to be maintained with high precision. Furthermore, a cleaning process is required to be performed on an upstream side of the photoreceptor and the charging roller in order to prevent the photoreceptor or the charging roller from being scratched or contaminated. The cleaning process potentially causes an increase in load torque, or abrasion of, and scratches on, the surface of the photoreceptor.
A feature of the invention is to offer an image forming apparatus using the noncontact charging method, capable of precisely adjusting a gap between a non-contact charging roller and a photoreceptor, so that the photoreceptor is prevented from being nonuniformly charged because of abnormal discharge or insufficient charging and therefore high quality image is ensured.